This invention relates to an arrangement for mounting a sparkplug of an internal combustion engine, wherein the sparkplug is used for detecting an ionic current flowing in a combustion chamber to detect an occurrence of knock or the like.
Conventionally, in an internal combustion engine for an automobile, the occurrence of knock or a limit of lean burn driving is judged by making use of an ionic current flowing in a combustion chamber immediately after ignition. The ionic current is detected by applying high voltage between a central electrode and an outside electrode of a sparkplug and detecting a current due to an electrical discharge with the central electrode and the outside electrode. In this case, in order to obtain the ionic current, positive voltage is applied to the central electrode of the sparkplug and negative voltage is applied to the outside electrode of the sparkplug. This arrangement is used to detect a subtle ionic current accurately by charging the outside electrode with negative electricity so as to charge the whole inner wall in the combustion chamber with negative electricity, which substantially increases an area of the electrode, which attracts a large amount of ions charged with positive electricity.
As a sparkplug for detecting an ionic current it is known, for example, that a surface area of a central electrode may be made wider, as mentioned in Japanese Laid Open Patent Publication No. 5-118266 or that a central electrode and an outside electrode arranged around the central electrode may be made longer than ordinary, as mentioned in Japanese Laid Open Patent Publication No. 9-317618, in order to improve an ionic current detection accuracy.
In the case where a central electrode and an outside electrode of a sparkplug are used as electrodes for detecting an ionic current, each of the electrodes suffers from abrasion due to electrical discharge of high voltage. In such situations, it is preferable to use a multi-pole sparkplug having a plurality of outside electrodes in order to improve resistance to abrasion. However, for a two-pole sparkplug having two outside electrodes each facing across a central electrode as the center, the accuracy of detecting an occurrence of knock may be lowered depending on a mounting direction of the outside electrodes on the cylinder head.
Generally, when driving after warming-up, the temperature of the inner wall of the combustion chamber near the exhaust valve becomes high. When mixed air touches the inner wall of the combustion chamber at high temperature, the mixed air may be ignited by itself due to the high temperature of the inner wall, resulting in an occurrence of knock. When drawing in and compressing air into the cylinder, the mixed air from the intake port passes near the exhaust port, which is arranged to face the intake port, and then passes an upper part of the piston and finally returns to the intake port again so as to circulate in the cylinder. More specifically, the mixed air is introduced from the intake port along a ceiling of the combustion chamber, flows along an inner face of the cylinder facing the intake port, and then flows along an upper part of the piston and the inner face of the cylinder so as to circulate in a cylinder. Due to the flow of the mixed air, the mixed air at the side of the intake valve burns later than the other portion that is ignited, and pressure at the intake valve side also rises. Then the temperature of the mixed air rises while combustion is retarded, which makes the portion of the mixed air where combustion was retarded ignite by itself, resulting in an occurrence of knock. As mentioned above, when knock occurs, a pressure wave is generated in the combustion chamber. If a plurality of outside electrodes and the central electrode fall on a line in a direction at generally a right angle with the pressure wave generated when knock occurs, the outside electrodes disturb diffusion of the ionic current, which may disturb detecting the ionic current.
An object of this invention is to solve the above problems. In order to attain the object, this invention takes the following measures. In an arrangement for mounting a sparkplug of an internal combustion engine, a sparkplug having a plurality of outside electrodes arranged to surround a central electrode is mounted so that the outside electrodes are arranged at positions to avoid a position generally at a right angle with respect to a direction in which a pressure wave travels. Alternatively, in the case where one of the outside electrodes is arranged at a position generally at a right angle with respect to a direction in which a pressure wave travels, the other remaining outside electrodes are arranged at positions to avoid facing each other across the central electrode.
This invention is an arrangement for mounting a sparkplug of an internal combustion engine wherein the arrangement detects an ionic current which flows after ignition in a combustion chamber of an internal combustion engine. An intake port and an exhaust port open into the combustion chamber. The ionic current is measured by the use of a sparkplug comprising a central electrode and a plurality of outside electrodes arranged spaced apart to surround the central electrode. The sparkplug is mounted so that all of the outside electrodes are arranged in areas at an intake port opening side of the cylinder (defined between the sparkplug and the intake port opening) and/or at an exhaust port opening side of the cylinder (defined between the sparkplug and the exhaust port opening). Alternatively, in the case where at least one of the outside electrodes is located outside of the above-mentioned areas, the other remaining outside electrodes are arranged at positions to avoid facing the outside electrode located outside of the areas described above across the central electrode.
In accordance with the arrangement, the outside electrodes are not arranged facing each other across the central electrode in a direction at generally a right angle with respect to the pressure wave generated when knock occurs. In other words, knock occurs when abnormal combustion occurs due to self-ignition in the intake port and the exhaust port. The pressure wave generated by the abnormal combustion travels or diffuses from the intake port opening to the exhaust port opening or from the exhaust port opening to the intake port opening. In the case where the outside electrode and the central electrode are arranged in a limited area existing in a direction the pressure wave travels or where they are arranged outside of the limited are abut other remaining outside electrodes are not located at a position facing the outside electrode across the central electrode, influence of the pressure wave on the ionic current is suppressed to a minimum. As a result, the ionic current can be detected with ease and the accuracy of the ionic current detected can be improved, thereby improving the accuracy of detecting an occurrence of knock.
For a sparkplug having a plurality of outside electrodes, it is preferable if all of the outside electrodes are arranged at positions not facing each other across the central electrode. For example, this may be accomplished by providing three outside electrodes arranged each spaced apart at an angle of 120 degrees around the central electrode as the center. In accordance with the arrangement of the outside electrodes according to the invention, in the case where at least one of the outside electrode is arranged at the position to be avoided, there is no other outside electrode at a position facing this outside electrode across the central electrode, which makes it possible to position the outside electrodes with ease when mounting the sparkplug.